Revolving measuring devices for concentration, shearing force, viscosity, dryness, rheological characteristics etc. have been in use for a long time in the process industry. The measuring devices are engineered after well established principles that in some cases date back to the 1960's. The fundamental principle is that a probe is rotated in a medium and the resistance torque that arise at the probe or sensor, as it is frequently called, is measured and transformed into a suitable out signal.
The most frequently occurring design is the principle with a dual axle system which measures the angular difference between two concentric axles, wherein the outer hollow axle is propelled with a constant rotational speed and the inner measuring axle is elastically connected to the hollow axle. The elastic connection also serves as sealing against the process medium. The probe is fixedly attached to the part of the measuring axle situated in the medium. The dual axle principle eliminates problems with additional torque, in the form of friction from the sealing and the bearing of the measuring axle, which could affect the measuring result, since this friction might vary. Said method is in control engineering usually referred to as the principle of motion balance. Disadvantages that occur with such a system are that temperature, pressure, and ageing of the material negatively affects the characteristics of the system, since the elastic sealing between the hollow axle and the measuring axle is allowed to operate outside of its zero position. Another characteristic that is negatively affected is the linearity of the system.
However, if the axle arrangement is equipped with a feedback system, which restores the inner axle to its zero position regardless of the size of the arisen torque and measures the force required, the effect of said disturbing factors are to a large extent eliminated and the result will be a more linear and long term stable measuring system. Such a feedback system is said to operate in accordance with the principle of balance of forces. Today, the most common way to apply feedback to a system for torque measuring device is through an electromagnetic feedback system, wherein the current required to retain the measuring axle in its zero position is measured.
Even though feedback systems are not novel in the area of torque measuring devices and even though they continuously have evolved since the 60's there are some existing disadvantages that remain hard, not to say impossible to eliminate with currently available technology. The use of electromagnetic systems always result in some remanence, which in turn result in a measurement error. Considerable improvements have been made with regard to this the last couple of years, but it is in the nature of the subject that obtaining zero remanence is impossible using available technology. Furthermore, said continuous development has not notably reduced the complexity and price of the measuring devices and even if the total weight has been reduced it is desirable that the weight is further reduced. The linearity has been improved, but also here one is dependent of the magnetization curve of the iron in use, which results in some nonlinearity.
In today's process industry, with high demands on quality and equally high volumes of production, one is in many measurement positions completely dependent on measuring devices with high accuracy. Unfortunately measuring devices which do not comply with these demands are sometimes selected. The reasons for this can be many, for example high prices, high weights and large complexity of the measuring devices with high accuracy. Hence there is a lot to gain if the measuring devices can be further improved at the same time as the accuracy is increased, the weight is reduced and the prices is, if not lowered so at least kept on the same level.